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8/2/2019 Except Hand

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Exception Handling

Exception Definition

Exception Occurrence

Exception Handling

Exception Propagation


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Exception Error occurred in execution time

Abnormal termination of program

Wrong execution result

Provide an exception handling mechanism inlanguage system

Improve the reliability of application program

Allow simple program code for exeception checkand handling into source


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Treat exception as an object

All exceptions are instances of a class extendedfrom Throwable class or its subclass.

Generally, a programmer makes new exceptionclass to extend the Exception class which issubclass of Throwable class.


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class UserErr extends Exception { }class UserClass {

UserErr x = new UserErr();

// ...if (val < 1) throw x;

}


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We can pass the object which contains a message for

the exception in string form

[UserException.java]

class UserErr extends Exception {UserErr(String s) super(s); // constructor

}class UserClass {

// ...

if (val < 1) throw new UserErr("user exception throw message");}


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Hierarchical Structure of

Throwable Class

Object

Throwable

Error Exception

RuntimeException

...

... ...


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Definition of Exception

Error Class

Critical error which is not acceptable in normalapplication program

Exception Class

Possible exception in normal application programexecution

Possible to handle by programmer


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System-Defined Exception

Raised implicitly by system because of illegalexecution of program

When cannot continue program execution any

more

Created by Java System automatically

Exception extended from Error class and

RuntimeException class

[DivByZero.java]


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System-Defined Exception

IndexOutOfBoundsException : When beyond the bound of index in the object which use index,

such as array, string, and vector

ArrayStoreException :

When assign object of incorrect type to element of array NegativeArraySizeException :

When using a negative size of array

NullPointerException : When refer to object as a null pointer

SecurityException : When violate security. Caused by security manager

IllegalMonitorStateException : When the thread which is not owner of monitor involves wait or

notify method


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Programmer-Defined Exception

Exceptions raised by programmer

Check by compiler whether the exception

handler for exception occurred exists or not If there is no handler, it is error

Sub class of Exception class


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Raised implicitly by system

Raised explicitly by programmer

throw Statement

[ThrowStatement.java]

throw ThrowableObject;

Throwable class or

its sub class


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class ThrowStatement extends Exception {public static void exp(int ptr) {

if (ptr == 0)throw new NullPointerException();

}public static void main(String[] args) {

int i = 0;ThrowStatement.exp(i);

}}

java.lang.NullPointerExceptionat ThrowStatement.exp(ThrowStatement.java:4)at ThrowStatement.main(ThrowStatement.java:8)


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throws Statement

When programmer-defined exception is raised, ifthere is no exception handler, need to describe it in

the declaration part of method

[ThrowsClause.java]

[modifiers] returntype methodName(params) throws e1, ... ,ek { }


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Exception Handling

try-catch-finally Statement

Check and Handle the Exception

[ExceptionHandler.java]

try {

//

} catch (ExceptionType1 identifier) {

//

} catch (ExceptionType2 identifier) {

//

} finally {//

}


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Exception Handling

Default Exception Handler

When system-defined exception occurred, if

programmer does not deal with it, it would be

processed by default exception handler

Simple function to output error message and exit

Execution Order of Exception Handler

Finally clause is executed independent of exceptionand catch

[SystemHandler.java] , [FinallyClause.java]


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Manage exceptions by collecting them in a specificmethod to propagate the exceptions to calling method

To prevent scattering of exception handling

Exception Propagation Order

If there is no catch block to deal with the exception, it ispropagated to calling method

All executions are ignored until finding the exceptionhandler

[Propagate.java]


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public class Propagate {void orange() {

int m = 25, i = 0;i = m / i;

}void apple() {

orange();}public static void main(String[] args) {

Propagate p = new Propagate();p.apple();

}}

Output by Default Exception

Handler

ArithmeticException Occurred

java.lang.ArithmeticException: / by zeroat Propagate.orange(Propagate.java:4)at Propagate.apple(Propagate.java:8)

at Propagate.main(Propagate.java:11)


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Explicit Description for possibility of ExceptionOccurrence

System-Defined Exception Do not need to announce the possibility of exception

occurrence

Programmer-Defined Exception When it is not managed in correspond method, the

exception type should be informed.

Use the throws clause


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[MsgException.java]

class MyException extends Exception { }

public class ClassA {

//

public void methodA() throws MyException {

// if (someErrCondition())

throw new MyException();

//

}

}


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Summary of Exception Handling

Objectives

Making safer program by providing special mechanism

Situation of Exception Handling

Error Correction and re-calling of the method whichoccurred the exception

Error Correction and continued execute without re-calling of method

Alternative way instead of giving up the execution result

After dealing with the exception, re-occur the same/otherexception to caller

Exit the program when raising the exception

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